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Re: [TCML] MOT Measurements

Hi All,

I hope I'm not boring you with this (I'm sure some are). But I think the next step is to look at a 15/30 NST which has not been modified. I have a couple on hand (Franceformers). I have a "gut feeling" that we really don't know where shunted transformers (NST's, MOT's, etc.) are truly at. Some resonant charging may or may not be occurring with our tank cap values. All the programs (including my own Javatc) are looking at Cres of non-shunted transformers. The shunted transformer is a different beast and without measurements, only guesses.

These transformers force leakage inductance via the shunts to apply regulation. This changes the transformer as we assume it to be as far as currents, voltages, Cres, etc. The load also changes the output. NST's will begin to suffer a voltage drop even with a small load. MOT's are current beasts, and they will require larger loads before significant change in output voltage is seen. Load impedance of shunted transformers is yet another set of tests that should be performed at some point.

I'll run through my normal 10V increment routine with one of my 15/30's. I'll set a variac to as near 10V as I can get, then measure the open and short circuit primary currents as well as open secondary voltage. Then head up to 20V and do all again. This is my routine as it keeps the source voltage unchanging, and thus, the numbers for Ioc, Isc, Vin, and Vout are all in very good agreement. My plan is to first do this unloaded (basic transformer testing). But loading is yet another issue.

If anyone has any additional thoughts, please let me know. For the NST, considering it's low power, I can will also measure true power as well as VA.

Best regards,

bartb wrote:
For an unmodified NST (shunts in tact), I expect Cres to be somewhat near what we would expect (maybe), but Cres will change depending on magnetizing current and current through the magnetic shunts. The shunts have inductance with the AC cycle, and as current is increased, the leakage inductance is increased. The result is a rather significant changing reactance dependent ultimately on the input voltage, and thus Cres will not be a constant throughout a voltage range (such as driving an NST with a variac). The impedance of a shunted transformer is not constant with varying input voltages (which is counterintuitive to non-shunted power transformers).

Best regards,

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